Prepare and roll out Pastry for Single-Crust Pie. Line an 11-inch tart pan with a removable bottom with pastry. Press pastry into fluted sides of tart pan and trim edges. Do not prick pastry.
For filling, in a large bowl beat eggs slightly with a fork. Stir in corn syrup. Add brown sugar, melted butter, and vanilla, stirring until sugar is dissolved. Stir in nuts and the 12 cup chocolate pieces. Place pastry-lined tart pan on a baking sheet; place baking sheet on the oven rack. Carefully pour filling into tart pan. Bake pie in a 350 degrees oven about 40 minutes or until a knife inserted near the center comes out clean; cool on a wire rack.
To serve, cut tart into slices; transfer to dessert plates. If desired, in a small heavy saucepan melt the 13 cup chocolate pieces and shortening over very low heat. Immediately remove from heat; stir until smooth. Cool slightly. Drizzle over tart slices (see tip, page 247). Cover and chill remaining tart for up to 2 days.
Pastry for Single-Crust Pie
Stir together flour and salt. Using a pastry blender cut in shortening until pieces are pea-size. Sprinkle 1 tablespoon of the water over part of the mixture; gently toss with a fork. Push moistened dough to side of the bowl. Repeat moistening dough, using 1 tablespoon of the water at a time, until all the dough is moistened. Form dough into a ball. On a lightly floured surface, use your hands to slightly flatten dough. Roll dough from center to edge into a circle about 12 inches in diameter.